The "round or wrinkled" and "yellow or green" traits of pea seeds represent the simplest cases of Mendelian inheritance, where the diploid genotypes AA and Aa ( or the equivalent aA) give a dominant phenotype, and only the genotype aa gives a recessive phenotype. As we saw last time, if we start with true-breeding parents (one homozygous dominant, one homozygous recessive), all of the F1 progeny will be heterozygous and show the dominant phenotype, and then these will give rise to a 3:1 ratio of phenotypes in the F2 generation in a monohybrid cross, and to a 9:3:3:1 ratio of phenotypes in the F2 generation in a dihybrid cross.

Sometimes things are a bit more complicated . . . .

1. What is "Incomplete Dominance" and how can it alter the phenotypic outcomes of crosses?

Figure 3.20 shows that in some cases the visible phenotype of a heterozygote (Aa) is different from the phenotype of a homozygous dominant (AA). A classic example of this is flower color in snapdragon plants (AA plants have red flowers, Aa plants have pink flowers, and aa plants have ivory flowers).

2. How is the genetics of human ABO blood groups an example of "Codominance involving multiple alleles"?

The human ABO blood groups are determined by a single gene, designated I, that codes for a glycosyl transferase enzyme. Table 3.3 (page 115) shows the six different possible genotypes (column 1), resulting from the fact that there are three alleles of this gene in the human population. The A and B alleles are both dominant, and the O allele is recessive. Thus, the six diploid genotypes result in four phenotypes (Column 3: blood types A, B, AB, and O).
(For our purposes, you don't have to worry about columns 4, 5, and 6 in this table.)

3. What is meant by the term "Epistasis"?

Figure 3.23 shows a classic case of epistasis, which signifies some alteration in the basic 9:3:3:1 phenotypic ratio expected from a dihybrid cross. In the case shown, we are looking at a single phenotypic trait (pea flower color) that is determined by two independent genes. We get the dominant phenotype in plants that have at least one dominant allele of EACH of the two genes; otherwise we get the recessive phenotype. So, the observed ratio in the F2 generation is 9:7.

4. What is "Complementation", and what does a "complementation test" allow us to determine?

Complementation is a specific genetic term that refers to the situation shown in the top part of Figure 3.23 that we just considered. If the crossing of two homozygous recessive "mutant" strains (for the same phenotypic trait) gives F1's that all show the dominant phenotype, we say that the two mutants show complementation, and we can conclude that the mutations in the parents are in different genes.

Figure 3.25 shows how a "complementation test" can be done. Assume we have found three new mutant pea plants that have white flowers. We ask : "For any pair of plants, are the homozygous recessive mutations in the same gene or in different genes?" Figure 3.25 shows the result for the mutation in plants 1 and 2 being in the same gene, but the mutation in plant three being in a different gene. That is, a complementation test allows us to determine whether two mutant strains showing the same phenotype have their mutations in the same gene or in different genes.

Thus, in general, complementation analyses allow us to determine whether a certain trait is determined by just one gene or by more than one gene. Inherited deafness in humans is an example. Consider that for two genes (call them alpha and beta for now) that are involved in hearing, a person needs at least one A allele and one B allele to hear. A deaf woman of genotype aaBb and a deaf man of genotype Aabb can have a child with normal hearing (genotype AaBb).